Optical waveguides are incorporated in optical waveguide devices, optical integrated circuits, and optical wiring boards and are in extensive use in optical communication, optical information processing, and other general optical fields. In the case of such an optical waveguide produced on a substrate such as, e.g., silicon or quartz, it is constituted of a core layer which transmits light and one or more cladding layers which have a lower refractive index than the core layer. More specifically, examples thereof include an optical waveguide 1 of a three-layer structure formed on a substrate 2 as shown in FIG. 11. This optical waveguide 1 comprises an undercladding layer 11 formed on the substrate 2, a core layer 12 formed on the undercladding layer 11, and an overcladding layer 13 which surrounds the core layer 12.
There are cases where an optical waveguide 1 is used in a flexible form. In such cases, the optical waveguide 1 described above is formed on a substrate 2 and is used after it is cut into a given length on the substrate 2 and then peeled from the substrate 2.
In general, fluorinated polyimides, which are excellent in both heat resistance and light transmission, are used as a material for forming the two cladding layers 11 and 13 or as a material for forming the core layer 12. However, the undercladding layer 11 (fluorinated polyimide) in such constitution has considerably reduced adhesion to the substrate 2 (e.g., a silicon wafer). There is hence a problem that separation between the substrate 2 and the undercladding layer 11 occurs during cutting and the optical waveguide 1 moves, resulting in a distorted cut surface. In case where the optical waveguide 1 has an end surface having a distorted shape, this arouses troubles in light transmission.
A technique for enhancing adhesion between the undercladding layer of an optical waveguide and a substrate so as to facilitate the cutting of optical waveguides has been proposed (see, for example, patent document 1). This technique comprises using as a substrate a glass substrate having a surface coated with vapor-deposited copper and forming an optical waveguide over the surface through an adhesive. In this technique, the optical waveguide is cut into a given length and thereafter the glass substrate with the cut optical waveguide is immersed in dilute hydrochloric acid to dissolve away the vapor-deposited copper on the glass substrate surface and thereby peel the optical waveguide from the glass substrate.
Patent Document 1: JP 08-313747 A (Examples 1 and 2)
However, the technique disclosed in patent document 1 has drawbacks that the peeling operation is accompanied by a danger because hydrochloric acid, which is a strong acid, is used for peeling and that workability is poor.